Because of the explosive growth in Internet traffic in recent years, transmission capacities between nodes have been increased by employing light for transmissions performed between nodes. For this, a large channel capacity is realized by exploiting one of the features of light, i.e., low loss. Further, for short distance transmission, performed between boards or racks that have been mounted in a data communication apparatus, the replacement of electrical wires has been begun by utilizing the high-speed transmission feature. Furthermore, bottlenecks are pointed out for electrical wiring used between LSI chips and on LSI chips, and a feasibility study for the provision of optical fiber cables has been begun.
A microcavity laser is a laser having a micron-order size that is aimed at large-scale integration of a photonic integrated circuit or an LSI that is employed for the above described application.
As such a microcavity laser, a pumped microcavity laser having a photonic crystal resonator has been proposed, as in non patent literature 1. The enlarged image, taken by an electron microscope, of a H0 nanolaser shown in FIG. 1(b) in non patent literature 1, is shown in FIG. 18A, a mode intensity characteristic and a laser spectrum along a laser threshold, which are illustrated in FIG. 2 (c) in non patent literature 1, are shown in FIG. 18B, and a relationship between normalized pump power and an intensity, illustrated in FIG. 2(d) in non patent literature 1, is shown in FIG. 18C. According to this, continuous laser oscillation at room temperature has been observed by employing a two-dimensional photonic crystal slab that is one of the photonic crystal resonator types. However, although a clear threshold value can be seen, the laser oscillation is confirmed only until a point immediately above the threshold value. This is due to the following reasons. Since the volume of an active layer (core layer) is two digits or more smaller for a microcavity laser than for an ordinary laser, light confinement efficiency for the resonator must be increased in order to obtain continuous oscillation at room temperature. Therefore, a large difference in a refractive index between a semiconductor and air is employed for a two-dimensional photonic crystal slab. As a result, efficient light confinement can be provided and continuous oscillation at room temperature can be obtained. However, since the thermal conductivity of air is extremely low, heat generated at the active layer can not effectively be radiated. Therefore, when the excitation intensity is increased, the rise of the temperature of the active layer occurs and halts oscillation, and a high light intensity can not be obtained. Furthermore, in order to provide efficient light confinement, it is important that a fluctuation in the thickness of the two-dimensional slab, sandwiched by air, should be reduced. Therefore, an epitaxial growth substrate having the same composition is employed for the entire area across which the beam of a microcavity laser is spreading. However, when this epitaxial substrate of the same composition is employed, carriers generated by light excitation are diffused isotropically to the outside of the resonator, so that the excited carriers can not be efficiently converted into those to be used for laser oscillation, and an increase in the threshold value and a reduction in the output intensity occur.
The photonic crystal resonator can be employed also as an optical switch by using a change in a refractive index involved in the excitation of carriers in the photonic crystal. For example, in non patent literature 2, carriers are generated in a resonator included in a photonic crystal, and a change in a transmittance involved in modulation of the refractive index of the resonator is employed as an optical switch.
However, since the confinement of carriers is not taken into account in this case, as well as in the above described laser, carriers excited in a very small space are rapidly diffused, and a device having extremely low efficiency is provided. Further, since with this structure heat can not be easily dissipated, heat that is generated upon carrier relaxation is accumulated, and a rise of temperature due to the accumulation of heat causes deterioration of the device characteristics.